1. Field of the Invention
The present invention generally relates to field effect transistors for integrated circuits and, more particularly, to inexpensive, ultra-low power dissipation devices suitable for extremely high integration density and operation at high clock speeds which can be reliably fabricated with low process cost and minimal process complexity.
2. Description of the Prior Art
Increasing integration density in current integrated circuit designs of limited chip area has been driven by the potential for improved performance at increased clock speeds because of reduced signal propagation time and increased noise immunity resulting from increased proximity between electronic devices integrated on a semiconductor chip as well as the potential for increased functionality on a single chip while reducing process cost per device on a chip. However, since each electronic device, and active devices such as transistors, in particular, must dissipate some power at each switching transition and the amount of heat which can be dissipated from a chip of limited area at a temperature which does not produce damage in the chip is limited, it is power concerns which limit the performance of current integrated circuit designs, particularly microprocessors, and may even preclude further performance
increases in the future.
More specifically, an ideal transistor would dissipate no power when it is in a conductive state since it would exhibit no resistance and would dissipate no power when it is in a non-conductive state because it would carry no current. However, even in an ideal transistor, power would be dissipated in the short interval during switching between conductive and non-conductive states and vice-versa since both current and resistance would be non-zero during such intervals. In practical devices, power is also dissipated during the conductive and non-conductive states as well since there will be some leakage current in the non-conductive state and a substantial resistance will be exhibited in a conductive state.
Therefore, it has generally been the practice in digital switching circuits to operate transistors with control voltages well above their switching thresholds (sometimes referred to as super-Vth) in order to achieve the highest possible ratio between so-called on-resistance and off-resistance (sometimes referred to as the on/off ratio or off/on resistance ratio) to minimize power dissipation during on and off periods as well as to achieve maximum noise immunity and signal voltage swing. However, the off/on resistance ratio is invariably reduced when transistor designs are scaled to smaller sizes to accommodate increased integration density and many modern integrated circuits operate with off/on resistance ratios as low as 100:1 or even less even when driven with signals well above the switching threshold Vth (e.g. super-Vth voltages) and highly sophisticated transistor designs have been developed to maintain even those resistance ratios to allow currently available minimum lithographic feature sizes to be exploited. In particular, so-called extension and halo implants, which may be complex in geometry and which generally overlap the gate, have been used to reduce conduction channel resistance while minimizing short-channel and other deleterious effects although the overlap causes increase in gate capacitance and some reduction in switching speed and increase of drive current requirement. Such reduction in switching speed and increase in gate capacitance are deemed to be tolerable to obtain improved off/on resistance ratio.
In this regard, so-called external resistance (e.g. the extrinsic series resistance in the source and drain and connections thereto at the ends of the transistor conduction channel) is an unavoidable component of the on resistance of the transistor and, although requiring expensive additional process steps (including annealing which may consume a significant portion of the overall manufacturing heat budget), silicidation (e.g. forming an alloy of the semiconductor material with a metal or combination of metals) of the gate and source and drain regions has been employed in many modern transistor designs in order to minimize resistance therein although such reduction in resistance also tends to slightly increase the gate to source/drain capacitance in some cases, particularly at the edges of the gate and especially where the source/drain and/or extension implants overlap the gate. Additionally, transistors having unsilicided source and drain regions were found to operate only poorly, if at all, in super-Vth voltage regimes without overlap of the gate with source and drain regions or at least the extension implants due to the significant contributions to extrinsic series resistance of both the bulk semiconductor between the gate and the source/drain diffusions which the gate thus cannot control and the resistance of the source/drain regions themselves. Thus, the overlap capacitance was substantially unavoidable, especially without silicidation of the source and drain regions, and the process complexity incident thereto.
Such designs may often only be realized by complex and costly process sequences with significant loss of manufacturing yield and which may also limit the minimum sizes to which such designs may be scaled while the devices so fabricated may still be limited in performance by the ability to remove heat therefrom. Complex and costly cooling arrangements such as micro- and nano-structure fans, forced air or liquid cooling and the like have often been used to support marginal performance improvements for critical applications while heat dissipation remains a major limitation on obtaining the full theoretical performance levels of current and foreseeable integrated circuit designs.
Other approaches to the problem have included reduction in operating voltages (which also allows some additional scaling of structures to dimensions where electrical breakdown would otherwise occur) while operating with super-Vth control voltages. However, circuit delays increase dramatically at reduced operating voltages due to reduced switching speed which may, in turn, limit the reduction in power dissipation which can be achieved (i.e. since the switching time then consumes a greater portion of a clock cycle, thus increasing the duty-cycle of relatively high power dissipation operation). However, at least functionally, some studies have shown that slow switching speed can be more than off-set in processing throughput by using a large degree of parallelism to achieve net gains in power limited performance although parallelism implies some increase in the overall numbers of active devices and size of logic circuitry; tending to reduce such a gain in power-limited performance, at least to some degree.
Therefore, it is seen that at the present state of the art, known improvements in integrated circuit design directed to avoiding limitation of performance by power dissipation have been largely exhausted and often do not allow the full potential performance of current integrated circuit designs to be achieved. Moreover, recent marginal improvements in integrated circuit performance have been achieved only by extremely aggressive transistor design which is of increasing incremental expense to fabricate, both in terms of process complexity and manufacturing yield, to realize diminishing increments of performance improvement and which are therefore becoming much less cost-efficient.